This invention concerns surfactant-enhanced gas flooding operations where a dilute surfactant solution and a gas are used to displace and recover hydrocarbons from within a subterranean reservoir. The invention is particularly applicable to gas flooding operations where carbon dioxide is injected in alteration with an aqueous fluid, such as water or brine.
A variety of techniques have been used to enhance the recovery of hydrocarbons from subterranean reservoirs in which the hydrocarbons no longer flow by natural forces. One such technique is water injection, or water flooding, to force hydrocarbons from the subterranean reservoir by flowing water through the formations. While water injection has been effective in many instances, only about a third of the hydrocarbons are recovered after application of this technique. The water bypasses much of the oil due to a combination of natural fractures, permeability variations, and fluid mobility differences. Trapping of oil by capillary forces at small pore throats also reduces recovery.
Another technique is the use of gas injection, which functions to force hydrocarbons from the subterranean formation. Gas flooding for oil recovery is frequently used subsequent to water flooding. A typical method of gas flooding utilizes recycled reservoir gases, reinjecting at the injection well at least a portion of those gases produced at the production well. To enhance the effectiveness of gas flooding, a miscible gas may be used to swell and reduce the viscosity of oil present in the formation. Carbon dioxide, which acts as a solvent to reduce viscosity of the crude oil, is one of the most effective, and least expensive, miscible gases. A gas flooding process which involves using carbon dioxide obtained by processing some of the crude oil produced from the reservoir is disclosed in U.S. Pat. No. 3,442,332.
Due to the low viscosity of gas, it will finger or flow through the paths of least resistance, thus bypassing significant portions of the formation, and causing early breakthrough at the production well. Also, due to its low density, the injected gas tends to rise to the top of the formation and "override" portions of the formation. The mobility of the injected gas, combined with variations in reservoir permeability, often results in an irregular injection profile, which provides lower hydrocarbon recovery efficiencies. A variety of techniques may be used to improve injection profile, and thereby to improve recovery efficiencies. A process for improving the gas injection profile, and the vertical sweep efficiency of a gas flooding operation, is described in U.S. Pat. No. 4,715,444.
The economics of recovering hydrocarbons using carbon dioxide can be greatly increased if the carbon dioxide is used in slug form and driven through the reservoir by an aqueous drive fluid, such as water, brine, or carbonated water. A process using this technique is disclosed in U.S. Pat. No. 3,065,790. Injection of carbon dioxide as a supercritical fluid has become the preferred method for carbon dioxide flooding operations, since it is more effective for oil recovery than carbon dioxide in the gaseous form. In a formation that has previously been water flooded, use of a carbon dioxide water-alternate-gas (WAG) injection process may provide additional oil recovery. However, only the oil actually contacted by the injected gas will be recovered, and bypassed oil will remain in the reservoir. A method to optimize well spacing and injection rates to achieve maximum recovery is described in U.S. Pat. No. 4,427,067.
It has been suggested that the overall efficiency of a gas or WAG flooding secondary recovery process can be improved by including a foaming agent or surfactant. The surfactant or foaming agent can be introduced directly into the reservoir by means of a water or brine vehicle prior to injection of the gas. The surfactant or foaming agent should have sufficient foaming ability and stability to satisfactorily reduce mobility of the gas, thereby reducing its tendency to channel through highly permeable fissures, cracks, or strata, and directing it toward previously unswept portions of the formation. These surfactants should be chemically and thermally stable under reservoir conditions. A process for improving the reservoir sweep efficiency of a gas flooding process by injecting a surfactant to form a foam in-situ is revealed in U.S. Pat. No. 3,342,256. The use of a polymer in combination with a surfactant, has also been proposed to improve the efficiency of a gas flooding operation, as is described in U.S. Pat. No. 4,676,316. Information on the use of surfactants in carbon dioxide flooding operations may be found in two papers published in 1985 by the Society of Petroleum Engineers: "CT Studies of Surfactant-Induced CO.sub.2 Mobility Control" by Wellington et al (SPE 14393) and "Surfactants for CO.sub.2 Foam Flooding" by Borchardt et al (SPE 14394).
Surfactant-induced carbon dioxide foam is a promising method for reducing carbon dioxide mobility in the reservoir. When this technique is utilized, foam will be formed in the more highly permeable strata within the immediate vicinity of the injection well, rather than in the less permeable zones, since the solution will take the path of least resistance. The carbon dioxide subsequently injected will more readily penetrate the previously less permeable strata because of the mobility reduction created by foam in the highly permeable zones. Carbon dioxide mobility can be adjusted through surfactant concentration and gas/liquid injection ratios.
A primary disadvantage of using surfactants to affect a large reservoir in a gas flooding operation is the substantial cost of surfactants. Another disadvantage is the reduction in fluid injectivity which results if a strong foam is formed in the reservoir, and the consequent delay in oil production. It is an object of this invention to provide an improved surfactant-enhanced gas flooding oil recovery process by which greater quantities of oil can be produced faster, with smaller amounts of surfactants. Another object of this invention is to provide an improved surfactant-enhanced gas flooding process which achieves sweep efficiencies comparable to those described in typical surfactant-enhanced gas flooding operations while maintaining adequate fluid injectivity. It is another object of this invention to more effectively utilize the surfactant used in a surfactant-enhanced gas flooding enhanced oil recovery process.